1. Field of the Invention
The present invention relates to a magnetoresistive element used in a magnetic head, a magnetic random access memory (MRAM), etc. and a nonvolatile memory including the magnetoresistive element.
2. Description of the Related Art
A multilayer film with a basic structure of ferromagnetic layer/non-magnetic layer/ferromagnetic layer can provide magnetoresistance effects by allowing a current to flow across the non-magnetic layer. When the non-magnetic layer is a tunnel insulating layer, a spin tunnel effect can be obtained. When the non-magnetic layer is a conductive metal such as Cu, a CPP (current perpendicular to the plane) GMR effect can be obtained. These magnetoresistance effects depend on the magnitude of a relative angle between magnetizations of the ferromagnetic layers that sandwich the non-magnetic layer.
For a magnetic memory device using the magnetoresistance effects, information is written by changing the magnetization direction of a ferromagnetic layer that serves as a storage layer, and the information is read by detecting the resultant change in resistance. Generally, a “hard” magnetic material having a large coercive force or a magnetic material that is “pinned” by laminating an antiferromagnetic layer is used for the ferromagnetic layer to correlate the storage layer with the relative angle between magnetizations. Thus, the magnetization direction of this ferromagnetic layer cannot be affected easily by an external magnetic field. A “soft” magnetic material whose magnetization direction is changed under a weak magnetic field is used for the storage layer. A magnetic field generated by a current flowing through a conductor that is located quite close to the storage layer changes only the magnetization direction of the storage layer to write information.
When a magnetoresistive element is used in a device, particularly in MRAM or the like, a monolithic structure combining the magnetoresistive element and a conventional Si semiconductor is essential in view of the cost and the degree of integration. The manufacturing process of the Si semiconductor includes heat treatment at high temperatures, e.g., heat treatment in hydrogen at 400° C. to 450° C. to remove wiring defects. However, the magnetoresistance characteristics of the magnetoresistive element are degraded by heat treatment at about 300° C. to 350° C. Even if the magnetoresistive element has high heat resistance, the degradation occurs at 300° C. to 380° C. or more.
A method in which a magnetoresistive element is incorporated after the formation of a semiconductor element also has been proposed. However, this method requires that wiring or the like for applying a magnetic field to the magnetoresistive element should be formed after producing the element. Accordingly, a variation in wiring resistance cannot be suppressed without the heat treatment for defect removal, thus reducing the reliability and stability of the element.
The wiring resistance variation can be suppressed easily by increasing a signal output from the magnetoresistive element. Therefore, it is desirable to make the rate of change in magnetoresistance (MR ratio) of the magnetoresistive element as high as possible.